The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a wireless device such as a user equipment (UE) 150 is wirelessly connected to a base station (BS) 110a commonly referred to as an evolved NodeB (eNodeB), as illustrated in FIG. 1a. Each eNodeB 110a-c serves one or more areas each referred to as cells 120a-c, and are connected to the core network. In LTE, the eNodeBs 110a-c are connected to a Mobility Management Entity (MME) 130 in the core network. A positioning node, also called a location server, may be connected to the MME 130. The positioning node is a physical or logical entity that manages positioning for a so called target device, i.e. a wireless device that is being positioned, and is in a control plane architecture referred to as an Evolved Serving Mobile Location Center (E-SMLC) 140. As illustrated in FIG. 1a, the E-SMLC 140 may be a separate network node, but it may also be a functionality integrated in some other network node. In a user plane architecture, the positioning is a part of a Secure User Plane Location (SUPL) Platform (SLP).
LTE Positioning Protocol (LPP) and LTE Positioning Protocol annex (LPPa) are protocols used for carrying out positioning in the control plane architecture in LTE. LPP is also used in the user plane architecture, whilst LPPa may be used to support user plane positioning. When receiving a positioning request, the E-SMLC may request positioning related parameters from eNodeB via LPPa. The E-SMLC then assembles and sends assistance data and the request for the positioning to the target wireless device, e.g. the UE, via LPP. FIGS. 1b-c illustrate example architectures and protocol solutions of a positioning system in an LTE network. In the control plane solution, illustrated in FIG. 1b, the UE communicates with the E-SMLC transparently via the eNodeB and the MME over LPP, and the eNodeB communicates with the E-SMLC transparently via the MME over LPPa. The user plane solution illustrated in FIG. 1c does not rely on the LPPa protocol, although 3GPP allows for the possibility of inter-working between the control and user plane positioning architectures. The SLP is the positioning node for user-plane positioning, similar to E-SMLC for control plane positioning, and there may or may not be an interface between the two positioning servers.
UE positioning is a process of determining UE coordinates in space. Once the coordinates are available, they may be mapped to a certain place or location. The mapping function and delivery of the location information on request are parts of a location service which is required for basic emergency services. Services that further exploit location knowledge or that are based on the location knowledge to offer customers some added value are referred to as location-aware and location-based services. The possibility of identifying a wireless device's geographical location in the network has enabled a large variety of commercial and non-commercial services, e.g., navigation assistance, social networking, location-aware advertising, and emergency calls. Different services may have different positioning accuracy requirements imposed by an application. Furthermore, requirements on the positioning accuracy for basic emergency services defined by regulatory bodies exist in some countries. An example of such a regulatory body is the Federal Communications Commission regulating the area of telecommunications in the United States.
In many environments, a wireless device position such as a UE position may be accurately estimated by using positioning methods based on Global Positioning System (GPS). Nowadays, networks also often have a possibility to assist wireless devices in order to improve the device receiver sensitivity and GPS start-up performance, as for example in an Assisted-GPS (A-GPS) positioning method. GPS or A-GPS receivers, however, may not necessarily be available in all wireless devices. Furthermore, GPS is known to often fail in indoor environments and urban canyons. A complementary terrestrial positioning method, called Observed Time Difference of Arrival (OTDOA), has therefore been standardized by 3GPP.
With OTDOA, a wireless device such as a UE measures the timing differences for downlink reference signals received from multiple distinct locations. For each measured neighbor cell, the UE measures Reference Signal Time Difference (RSTD) which is the relative timing difference between a neighbor cell and the reference cell. As illustrated in FIG. 2, the UE position estimate is found as an intersection 230 of hyperbolas 240 corresponding to the measured RSTDs. At least three measurements from geographically dispersed BSs 210a-c with a good geometry are needed to solve for two coordinates of the UE and the receiver clock bias. In order to find the position, precise knowledge of transmitter locations and transmit timing offsets is needed. Position calculations may be conducted, for example by a positioning node such as the E-SMLC or the SLP in LTE, or by the UE. The former approach corresponds to a UE-assisted positioning mode, and the latter corresponds to a UE-based positioning mode.
To enable positioning in LTE and facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning, such as positioning reference signals (PRS) have been introduced, and low-interference positioning subframes have been specified in 3GPP. Even though PRS have been specifically designed for positioning measurements and in general are characterized by better signal quality than other reference signals, the standard does not mandate using PRS. Other reference signals, e.g., Cell-specific Reference Signals (CRS) may also be used for positioning measurements.
PRSs are transmitted from one antenna port (R6) according to a pre-defined pattern, as described for example in clause 6.10.4 in 3GPP TS 36.211, v9.1.0, 2010-03-30. One of the currently agreed PRS patterns is shown in FIG. 3b, where the squares marked with R6 indicate PRS resource elements within a block of twelve subcarriers over fourteen OFDM symbols. Fourteen OFDM symbols correspond to a 1 ms subframe with normal cyclic prefix.
A set of frequency shifts may be applied to such a pre-defined PRS pattern to obtain a set of orthogonal patterns which can be used in neighbor cells to reduce interference on the PRS and thus improve positioning measurements. The effective frequency reuse of six can be modelled in this way. The frequency shift may be defined as a function of Physical Cell Identity (PCI) as follows:vshift=mod(PCI,6)in which vshift is the vertical frequency shift, mod( ) is the modulo function.
To improve hearability of the PRS, i.e., to enable detecting the PRS from multiple sites and with a reasonable quality, positioning subframes have been designed as low-interference subframes. It has thus also been agreed that no data transmissions are allowed in general in positioning subframes. As a result, synchronous networks' PRSs are ideally interfered with only by PRS from other cells having the same PRS pattern index, i.e., the same vertical frequency shift, and not by data transmissions.
In partially aligned asynchronous networks, PRS may still be interfered with by transmissions over data channels, control channels, and any physical signals when positioning subframes collide with normal subframes, although the interference is reduced by the partial alignment, i.e., by aligning the beginnings of positioning subframes in multiple cells within one-half of a subframe with respect to some time base.
PRS are transmitted in pre-defined positioning subframes grouped by a number NPRS of consecutive subframes, i.e. one positioning occasion, as illustrated in FIG. 3a. Positioning occasions occur periodically with a certain periodicity of N subframes, corresponding to a time interval TPRS between two positioning occasions. The standardized time intervals TPRS are 160, 320, 640, and 1280 ms, and the number of consecutive subframes NPRS are 1, 2, 4, and 6.
As PRS from multiple distinct locations need to be measured for OTDOA positioning, the UE receiver may have to deal with PRS that are much weaker than those received from the serving cell. Furthermore, without the approximate knowledge of when the measured signals are expected to arrive in time and what is the exact PRS pattern, the UE would need to do signal search within a large window which would impact the time and accuracy of the measurements as well as the UE complexity. To facilitate UE measurements, the network transmits assistance data to the UE, which includes, among the others, reference cell information, neighbour cell list containing PCIs of neighbour cells, the number of consecutive downlink subframes, PRS transmission bandwidth, and frequency.
PRS may be transmitted with a reduced power. This is also referred to as PRS muting. The muting should then apply for all PRS resource elements in all subframes of a PRS positioning occasion over the entire PRS transmission bandwidth. A PRS positioning occasion comprises of the consecutive downlink subframes containing the PRS signals. A PRS positioning can contain up to six consecutive downlink PRS subframes. The PRS signals transmitted from the neighboring cells can overlap. This means the PRS transmitted in an aggressor cell may cause interference to the UE when the UE performs PRS measurement such as RSTD measurements in a neighboring cell. The PRS muting is applied to the PRS transmitted in the aggressor cell. The purpose of the muting is thus to reduce the interference for a UE measuring the PRS in a cell which is neighbor to the aggressor cell. In a special case, PRS are transmitted with zero power, which means that they are simply not transmitted.
It is not specified by the standard how a PRS muting pattern is generated. Only the signaling means enabling the positioning node to configure a PRS muting pattern at the UE is specified. A muting pattern is thus decided by the network and may be signaled to the UE with the OTDOA assistance data. If the muting pattern is not signaled for a particular cell, the UE can assume that muting is not applied in this cell. If the muting pattern is signaled for a cell, the UE can assume that PRS are not transmitted in the corresponding cell in the positioning occasions which are muted. There are no restrictions on muting pattern configurations, i.e. the network has full flexibility to decide the pattern. For each cell where the muting pattern is applicable, the muting pattern configuration is signaled as a bit string, also called a muting sequence, where a bit which may have a value 0 or 1, indicates whether PRS is muted or not in the positioning occasion defined by the bit position in the bit string and the reference time point for the muting pattern. For example a pattern [00001111] associated with a cell with e.g. cell ID #10, means that the first four PRS positioning occasions are muted in cell #10. Muting is described in 3GPP TS 36.355, v9.2.1, Section 6.5.1.2, 2010-06-22. Sometimes muting sequence is used interchangeably with muting pattern.
Furthermore, RSTD accuracy requirements and RSTD measurement requirements defined in the 3GPP standard in 3GPP TS 36.133, v9.4.0, 2010-06-21, section 9.1.10 and 8.1.2.5-6, give the requirements for PRS in terms of a minimum PRS configuration such as a minimum number of positioning subframes per positioning occasion, and a minimum number M of positioning occasions for measuring at least n cells.
Several muting pattern solutions have already been mentioned or discussed in 3GPP at different levels of details. However, many of them are not applicable since they either do not take into account positioning measurement requirements, or are not meant for PRS muting over the entire positioning occasion. That the PRSs should be muted over the entire positioning occasion is a limitation that was recently introduced into the 3GPP standard. Two examples of known muting patterns are autonomous random muting patterns and PCI-based patterns.
With autonomous random muting patterns, the muting in cells is random. Each eNodeB decides whether PRS transmissions should be ceased or not, and the muting decision is made with some probability. In a simple implementation, there is no coordination among eNodeB's and the probability is statically configured per eNodeB or per cell. An advantage with random muting patterns is that no signaling is needed among eNodeB's. However, signaling of muting patterns over LPP has anyhow been recently introduced in the standard. A disadvantage with random muting patterns is that networks are typically inhomogeneous, with different cell coverage areas and user density and possibly with different types of BSs, which implies that a setting of optimal muting probabilities is as difficult as designing muting patterns in a planned and coordinated way. The primary objective of the PRS muting is to lower interference due to PRS transmission from an aggressor cell towards the victim UE when it performs PRS measurement in a neighbor cell. In heterogeneous deployment the cells operating at higher power, such as macro cells, are likely to have more adverse impact on the reception quality of signals received by the UE when it measures on PRS from low power neighboring cells, such as pico cells. The PRS muting based on random scheme does not guarantee that muting will be applied to the positioning occasions in all the aggressor cells. Hence random muting cannot fully guarantee the reduction of interference in network deployment especially where cells operate at different power levels. At the same time, designing muting patterns in a planned and coordinated way is likely to be more efficient than the random approach. This is because the network can ensure that the muting is applied in a selective manner to the PRS signals transmitted by all the aggressor cells causing significant interference to the UE.
PCI-based patterns imply designing a limited set of muting patterns and mapping the muting pattern identities to PCIs. An advantage with PCI-based patterns is that the UE may, given a table of muting patterns and the PCI received in the assistance information, find out when the PRSs are transmitted in the cell of interest without the muting information being explicitly signaled to the UE. However, as in the case of random muting patterns, this is less important since the signaling is allowed by the standard. A disadvantage with PCI-based patterns is that there is no flexibility as the configuration is based on a static cell planning. There is thus no possibility to re-plan a part of the network to address the positioning needs, and no possibility for planning and optimization in general, which is a big disadvantage in networks which are inhomogeneous by nature and may involve various types of BSs.
Hence, random muting patterns and PCI-based patterns are not flexible, and do not allow for planning and optimization of positioning performance. They are thus very difficult to adopt for inhomogeneous networks which are not planned specifically for positioning. Most of the conventional muting solutions are statically configured. Furthermore, in some conventional solutions it has been common to assume patterns that are cyclic shifts of each other, while in practice a network with cells of various sizes and different types of BSs would benefit from using different patterns, as has been captured in the random pattern solution where the probabilities are different for the different cells.
Muting and muting pattern may also in a more general way be referred to as a reduced transmission activity and a pattern for reduced transmission activity respectively, since muting can be viewed as a special case of reduced transmission activity. Reduced transmission activity patterns may be applicable not only for positioning muting, but also for interference coordination in heterogeneous networks, where reduced transmission activity schemes are adopted when creating blank or almost blank subframes.